Chloroquine reduces osteoclastogenesis in murine osteoporosis by preventing TRAF3 degradation Yan Xiu, … , Lianping Xing, Brendan F. Boyce J Clin Invest. 2014;124(1):297-310. https://doi.org/10.1172/JCI66947. Research Article Bone Biology The cytokines RANKL and TNF activate NF-κB signaling in osteoclast precursors (OCPs) to induce osteoclast (OC) formation. Conversely, TNF can limit OC formation through NF-κB p100, which acts as an inhibitor, and TNF receptor– associated receptor 3 (TRAF3); however, a role for TRAF3 in RANKL-mediated OC formation is unknown. We found that TRAF3 limits RANKL-induced osteoclastogenesis by suppressing canonical and noncanonical NF-κB signaling. Conditional OC-specific Traf3-KO (cKO) mice had mild osteoporosis and increased OC formation. RANKL induced TRAF3 degradation via the lysosome/autophagy system. The autophagy/lysosome inhibitor chloroquine reduced RANKL-induced OC formation and function by increasing TRAF3 expression in OCPs in vitro and in vivo. Although chloroquine had no effect on basal bone resorption, it inhibited parathyroid hormone– and ovariectomy-induced OC activation in WT, but not cKO, mice. Deletion of the transcription factor gene Relb resulted in increased TRAF3 expression in OCPs, which was associated with decreased RANKL-induced TRAF3 degradation. RelB directly increased expression of BECN1, a key autophagy regulator, by binding to its promoter. These data indicate that autophagic/lysosomal degradation of TRAF3 is an important step in RANKL-induced NF-κB activation in OCPs. Furthermore, treatments that increase TRAF3 levels in OCPs, including pharmacological inhibition of its degradation with compounds such as chloroquine, may limit bone destruction in common bone diseases. Find the latest version: https://jci.me/66947/pdf Research article Chloroquine reduces osteoclastogenesis in murine osteoporosis by preventing TRAF3 degradation Yan Xiu,1 Hao Xu,1 Chen Zhao,1 Jinbo Li,1 Yoshikazu Morita,1 Zhenqiang Yao,1,2 Lianping Xing,1,2 and Brendan F. Boyce1,2 1Department of Pathology and Laboratory Medicine, and 2Center for Musculoskeletal Research, University of Rochester Medical Center, Rochester, New York, USA. The cytokines RANKL and TNF activate NF-κB signaling in osteoclast precursors (OCPs) to induce osteoclast (OC) formation. Conversely, TNF can limit OC formation through NF-κB p100, which acts as an inhibitor, and TNF receptor–associated receptor 3 (TRAF3); however, a role for TRAF3 in RANKL-mediated OC forma- tion is unknown. We found that TRAF3 limits RANKL-induced osteoclastogenesis by suppressing canoni- cal and noncanonical NF-κB signaling. Conditional OC-specific Traf3-KO (cKO) mice had mild osteoporosis and increased OC formation. RANKL induced TRAF3 degradation via the lysosome/autophagy system. The autophagy/lysosome inhibitor chloroquine reduced RANKL-induced OC formation and function by increas- ing TRAF3 expression in OCPs in vitro and in vivo. Although chloroquine had no effect on basal bone resorp- tion, it inhibited parathyroid hormone– and ovariectomy-induced OC activation in WT, but not cKO, mice. Deletion of the transcription factor gene Relb resulted in increased TRAF3 expression in OCPs, which was associated with decreased RANKL-induced TRAF3 degradation. RelB directly increased expression of BECN1, a key autophagy regulator, by binding to its promoter. These data indicate that autophagic/lysosomal degrada- tion of TRAF3 is an important step in RANKL-induced NF-κB activation in OCPs. Furthermore, treatments that increase TRAF3 levels in OCPs, including pharmacological inhibition of its degradation with compounds such as chloroquine, may limit bone destruction in common bone diseases. Introduction double-KO mice (6, 7), but NF-κB–induced osteoclastogenesis Bone homeostasis is maintained by a delicate balance between by cytokines remains incompletely understood. NF-κB is a piv- osteoclastic bone resorption and osteoblastic bone formation. otal regulator of RANKL- and TNF-induced osteoclast precursor Disruption of this balance with a higher rate of bone resorption (OCP) differentiation and is activated via TNF receptor–associat- occurs in metabolic and inflammatory bone disorders, such as ed factors (TRAFs), which share a common C-terminal structural postmenopausal osteoporosis and RA, resulting in increased risk TRAF domain that mediates oligomerization and receptor bind- of fracture and joint destruction, respectively (1). Osteoclasts ing. RANKL activates both canonical and noncanonical NF-κB sig- (OCs) resorb bone in physiologic and pathologic conditions (2), naling in OCPs through TRAF6 and NIK, leading to activation of and their formation and activity are increased in osteoporosis IKKβ and IKKα, respectively (4, 8), and to stimulatory and inhibi- and RA as a result of increased production of proinflammatory tory signaling in the cells (5). Similarly, TNF stimulates OC for- cytokines, such as TNF and RANKL, which activate NF-κB (3). mation directly, but it also limits RANKL-and TNF-mediated OC NF-κB transcriptional factors regulate immune and numerous formation by increasing cellular levels of NF-κB p100 (9) and other other cellular responses and play critical roles in skeletal development, inhibitory proteins (4, 10). TNF typically induces fewer OCs than endochondral ossification, OC and osteoblast functions, and com- RANKL in vitro when WT OCPs are used, but it induces similar mon bone diseases (4). In mammals, there are 5 NF-κB family mem- numbers of OCs from Nfkb2–/– OCPs as RANKL and also robust bers: RelA (p65), RelB, and c-Rel, and the precursor proteins NF-κB1 OC formation in Rankl–/– and Rank–/– mice deficient in p100. (p105) and NF-κB2 (p100), which are processed into p50 and p52, These findings indicate that p100 inhibits TNF-induced osteo- respectively (5). Canonical NF-κB signals through IKKβ and NF-κB clastogenesis by a TNF receptor–associated receptor 3–dependent essential modulator (NEMO), leading to phosphorylation of IκBα (TRAF3-dependent) mechanism (9), but the mechanism whereby and translocation of p65/p50 heterodimers into nuclei in response TRAF3 inhibits osteoclastogenesis has been unclear, although it to cytokines; noncanonical NF-κB stimulates IKKα-mediated phos- does involve NIK degradation (11). phorylation of p100 through activation of NF-κB–inducing kinase Apart from their role as adaptor proteins, TRAFs also act as E3 (NIK) and leads to processing of p100 to p52 and release of RelB/p52 ubiquitin ligases, a function crucial for the activation of NF-κB complexes. Noncanonical signaling is induced by TNF family mem- signaling (12). Overexpression and genetic studies identified bers, including RANKL, CD40 ligand, BAFF, and lymphotoxin-β (5). positive activation roles for TRAF2, -5, and -6 in canonical NF-κB A role for NF-κB signaling in bone was first identified in reports signaling (13–15). However, study of the role of TRAF3 in NF-κB of defective cytokine-induced osteoclastogenesis in NF-κB1/2 signaling has been difficult because most Traf3-KO mice die within the first week after birth (16). A report that TRAF3 is constitutively Conflict of interest: The authors have declared that no conflict of interest exists. bound to NIK and mediates NIK ubiquitination and degradation Citation for this article: J Clin Invest. 2014;124(1):297–310. doi:10.1172/JCI66947. in B cells identified a critical negative regulatory role of TRAF3 The Journal of Clinical Investigation http://www.jci.org Volume 124 Number 1 January 2014 297 research article in noncanonical signaling (17). Furthermore, Traf3–/– mice have with TRAF3 having its major role in RANKL-induced OC forma- marked B cell accumulation of NIK, and compound deficiency of tion rather than function. NF-κB p100 rescued their early postnatal lethality (18). Although Importantly, we found that 2-month-old C-cKO mice have mild deficiency of NIK or its downstream signaling molecule, RelB, osteoporosis, as indicated by significantly reduced tibial trabecular appears to have little effect on basal bone homeostasis, Nik–/– bone volume (BV) (Figure 1D), trabecular number, and thickness, and RelB–/– mice have defective osteoclastogenesis in response to with mean values for trabecular connectivity and separation being pathologic osteolysis in vivo (8, 19). Additionally, transgenic mice numerically lower and higher, respectively, but these did not reach with OC-targeted overexpression of a mutant NIK lacking the statistical significance (Supplemental Figure 1D). These were TRAF3-binding domain have decreased bone mass and increased associated with increased OC numbers and surfaces (Figure 1E), bone erosion in a serum-transfer model of RA (11). Collectively, compared with control mice. these data indicate that NIK and TRAF3 play important negative Previous TRAF3 overexpression studies in B cells suggested regulatory roles in noncanonical NF-κB signaling. that TRAF3 suppresses canonical NF-κB activation (21). To test the mechanism by which TRAF3 inhibits osteoclastogenesis, we Results examined canonical and noncanonical protein levels in OCPs Mice with OC-specific deletion of TRAF3 have increased osteoclastogenesis overexpressing TRAF3 in response to RANKL and observed mark- and osteoporosis, mediated by increased canonical and noncanonical NF-κB edly increased expression of NFATc1, NIK, p52, RelB, and RelA signaling. Culture of WT BM cells with M-CSF for 2 days followed by in pMX-GFP–infected OCPs (Figure 1F). In RANKL-treated pMX- RANKL typically results in microscopically recognizable OCs